Aggression usually subsides after a short while. If one girl doesn't settle down you can find any container thats suitable for her. I personally prefer a breeder box so she and the other bettas can see each other. I kept one girl in the box for a week after she was continuously nipping everybody up. After that long week she settled down and is no longer trying to be alpha.

It's been proven bettas aren't territorial. They will fight so long as there is water involved.

It's their nature to fight regardless of territory. Females are more comunal which only means they can learn to become "accustomed" to one another. They're still intelligent enough to know when to fight or flight though - which is why having them in larger groups tends to reduce their aggression.

I don't rearrange anything in the tank when I add females. It seems to do nothing to help. Add more, add one, in any case the balance of aggression you created is disturbed and you just have to wait for things balance again.

What size is the tank? I'm sure I read it somewhere but i forgot, lol.

I've had successful sororities for a long time now so I would like to make a couple suggestions if you don't mind! First would you possibly be able to get some more plants? The more heavily planted a sorority is the better! They need well concealed hiding places to get away from eachother. Also 4 girls is the bare bare minimum number you should have, the fewer girls there are the more aggression there will be, female bettas do well slightly overstocked, do you plan on getting more girls?

It's been proven bettas aren't territorial. They will fight so long as there is water involved.

Proven where? You can't really do a fair experiment unless you're using WILD bettas. As far as I know, there aren't many animals that exist on earth who fight with their own kind on sight, and ignore all other aspects. A living creature's purpose in life is to create more life, and fighting to the death with its own species seems kind of backwards from a psychological standpoint.

If 6 is the max for a 10G, I would think 12 is the max for a 20G. Not sure how live plants or other species would imapct that. I have 6 bettas in my 20G and the 2 clown plecos. I have gotten very lucky - my females are pretty docile. Even the gender confused male in with them.

If you are having aggression issues, try adding more decor - like big, bushy plants.
When I had silk plants, alot of my plants were like this and some were taller then the tank. Taller ones will float at the surface and provide hiding and resting spots for them at night.
This kind: